The present invention relates broadly to a method of enhancing the photosensitivity of a photosensitive light transmissive material and to a method of creating an optical structure within a photosensitive light transmissive material. The present invention has applications in the creation of gratings and similar structures within optical waveguides, including in optical fibres, and the invention is hereinafter described in that context. However, it will be understood that the invention does have broader applications, including to the enhancement of the photosensitivity of various types of photosensitive light transmissive materials, and in various forms such as in planar form or in optical fibre form.
The creation of optical structures within photosensitive light transmissive materials, such as the writing of gratings in an optical fibre, is a significant process within optical technologies such as the technology of wavelength division multiplexing.
In the writing of gratings it is typically desirable to achieve high refractive index contrasts within a selected region of the optical fibre, the regions of different refractive index forming the optical grating structure. Whilst different techniques have been utilised to create photosensitive optical fibres or to enhance the photosensitivity of optical fibres, surprisingly the underlying chemistry and physics giving rise to both the photosensitivity and the ultimate refractive index changes exploited during the writing process remain poorly understood, even from a phenomenological viewpoint.
Therefore, developments in the area of photosensitising optical fibres have suffered in that they were primarily motivated through trial and error type experiments, some of which had the desired result of for example enhancing the photosensitivity in optical fibres.
As an example, it was found that hydrogen-loading of optical fibres facilitates the writing of stronger gratings that might otherwise be achieved. The hydrogen-loading is believed to have the effect of enhancing the photosensitivity of the optical fibre, particularly in those which exhibit relatively weak inherent photosensitivity. However, as mentioned above, the underlying chemistry and physics given rise to the enhanced photosensitivity have not been fully understood.
Also, it has been established that gratings that are written into hydrogen-loaded fibres exhibit a rapid initial decay which reflects in a poor long term stability of the high contrast gratings. Thus refrigeration based storage to maintain the high contrast gratings is required, and at the same time undesirable absorption peaks due to hydride-hydroxy absorption bands due to the hydrogen species remaining in the optical fibre have to be accepted.
It will be appreciated from the above that there is a need for alternative methods of enhancing the photosensitivity of photosensitive light transmissive materials and for creating optical structures in the photosensitive light transmissive material.
In accordance with a first aspect of the present invention, there is provided a method of creating an optical structure within a photosensitive light transmissive material comprising the steps of (a) exposing a selected region of the material to optical radiation at a wavelength selected to effect a refractive index change in the material ,(b) terminating the exposure to the radiation at a selected fluence; and afterwards (c) exposing at least one portion of the selected region to UV radiation at a level sufficient to vary the refractive index of the material within the selected region to from the optical structure; wherein the fluence is selected to render the optical structure substantially thermally stable at temperatures up to 250xc2x0 C. without a requirement for post-processing annealing.
It has been found by the applicant that the photosensitisation mechanism within a photosensitive light transmissive material is essentially a two-step process involving the successive formation of a first species in the material and of a second species in the material from the first species. The formation of the first species results in a first refractive index change in the material and the transformation into the second species results in a second refractive index change in the material. Having recognised the two-step nature of the photosensitisation process, the present invention in at least preferred embodiments can provide a method for an optimum pre-sensitisation of the photosensitive light transmissive material.
The term xe2x80x9cspeciesxe2x80x9d in not intended to be limited to be of a chemical nature but rather it can also, alternatively or in addition, be of a physical nature.
In one embodiment, the selected region comprises an intended optical grating region and the step of exposing the at least one portion of the selected region to the UV radiation comprises exposing periodic regions within the grating region to create a periodic grating structure within the region.
The method preferably further comprises the step of hydrogen-loading the selected region prior to the initial exposure to the radiation. It has been found by the applicant that hydrogen can act as a catalyst in the underlying process. In such embodiments, the method can preferably further comprise the step of removing the hydrogen-loading by out-diffusion after the initial exposure to the radiation.
The initial exposure of the material to the radiation can be controlled to be substantially uniform throughout the selected region. Alternatively, the initial exposure to the radiation can be controlled to be non-uniform to effect a non-uniform photosensitivity of the material throughout the selected region.
The wavelengths of the radiation or the UV-radiation can be about 244 nm or about 193 nm.
The radiation used in step (a) can be the UV-radiation of step (c).
The photosensitive light transmissive material may be in the form of an optical fibre.
In a second aspect, the formation of the optical structure may be effected in a two stage process, in which case the first stage is defined as providing a method of enhancing the photosensitivity of a photosensitive light transmissive material comprising the steps of (a) exposing a selected region of the material to optical radiation at a wavelength selected to induce a refractive index change in the material within the selected region; and(b) terminating the exposure to the radiation at a selected fluence; wherein the fluence is selected to render the exposed region substantially thermally stable at temperatures up to 250xc2x0 C. without a requirement for post-processing annealing after an optical structure is formed by exposing at least one portion of the selected region to UV radiation at a level sufficient to vary the refractive index of the material within the selected region.
Preferably, the method further comprises the step of hydrogen-loading the selected region prior to the exposure to the radiation.
Where the method comprises the step of hydrogen-loading the selected region prior to the exposure to the radiation, the method can further comprise the step of removing the hydrogen-loading by out-diffusion after the exposure to the radiation.
The exposure of the material to the radiation is preferably controlled to be substantially uniform throughout the selected region. Alternatively, the exposure to the radiation is controlled to be non-uniform to effect a non-uniform photosensitivity of the material throughout the selected region.
The wavelength of the radiation may be about 244 nm or about 193 nm.
The photosensitive light transmissive material may be in form of an optical fibre.
In accordance with a third aspect of the present invention, there is provided an optical structure formed in a photosensitive light transmissive material by a method as defined in the first aspect of the present invention.
In accordance with a fourth aspect of the present invention, there is provided a photosensitive light transmissive material having an enhanced photosensitivity achieved by a method as defined in the second aspect of the present invention.